Magnetization manipulation is the keystone for the magnetism based devices including traditional magnetic recording media and magneto-electronic devices, such as magnetic sensors, nonvolatile magnetic random access memories (MRAMs) and spin logic applications. In such devices, a magnetic layer is switched by a magnetic field. However, it is desirable to combine the nonvolatility of the magnetic bit with semiconductor microelectronics. Thus, the use of electric current, instead of magnetic field, to control magnetization is highly demanded.
Spin-transfer-torque (STT) was developed to switch the magnetization in a non-collinear magnetic system using spin-polarized current without the use of a magnetic field. For example, the racetrack memory and the STT-MRAM have been developed based on STT induced domain wall motion in magnetic nano-wires and the STT induced magnetization switch in giant magnetoresistance devices and magnetic tunnel junctions (MTJs), respectively. While STT based devices enjoy the benefit of site specific switching without magnetic field, the electrical current required to switch magnetization must passes through the device and must exceed a high threshold current density of greater than 107 A/cm2. This high value of current density is close to the breakdown limit of the devices, especially for MTJs. This drawback severely limits the viability of STT based devices.
To overcome the limitations described above, new technology has been proposed to utilize spin current generated from spin-orbit coupling (SOC) to manipulate the magnetic bit. A typical structure of such device comprise of Heavy-Metal/Ferromagnet bilayer. As an electric current passing through the Heavy-Metal, an effective torque will exert on the ferromagnetic layer, as so called spin-orbit torque (SOT). In this technique, the manipulation of the magnetic bit does not require high current density flowing in the FM layer, and therefore, promoted the lifetime and the reliability of the whole device. A shortcoming of such conventional techniques, however, is that they cannot manipulate a ferromagnetic layer with perpendicular magnetic anisotropy (PMA); i.e. its magnetic moments points in one of the two directions perpendicular to the film plane, without applying an in-plane magnetic field. While the PMA is generally considered necessary for achieving high recording density, the requirement of an external magnetic field is incompatible with microelectronics and magnetic storage. Such magnetic field requirement is undesirable and improved techniques. What is needed in the art, therefore, are improved devices and methods that avoid the limitations of conventional devices and methods.